Systems and Methods for Reducing Motion Blur in Images or Video in Ultra Low Light with Array Cameras

ABSTRACT

Systems and methods for reducing motion blur in images or video in ultra low light with array cameras in accordance with embodiments of the invention are disclosed. In one embodiment, a method for synthesizing an image from multiple images captured using an array camera includes capturing image data using active cameras within an array camera, where the active cameras are configured to capture image data and the image data includes pixel brightness values that form alternate view images captured from different viewpoints, determining sets of corresponding pixels in the alternate view images where each pixel in a set of corresponding pixels is chosen from a different alternate view image, summing the pixel brightness values for corresponding pixels to create pixel brightness sums for pixel locations in an output image, and synthesizing an output image from the viewpoint of the output image using the pixel brightness sums.

CROSS-REFERENCE TO RELATED APPLICATIONS

The current application is a continuation of U.S. patent applicationSer. No. 14/776,553, entitled “Systems and Methods for Reducing MotionBlur in Images or Video in Ultra Low Light with Array Cameras” toGabriel Molina, filed Sep. 14, 2015, which application is a 35 U.S.C.§371 National Stage Patent Application of PCT Patent Application SerialNo. PCT/US2014/025100 entitled “Systems and Methods for Reducing MotionBlur in Images or Video in Ultra Low Light With Array Cameras” toGabriel Molina, filed Mar. 12, 2014, which application claims priorityunder 35 U.S.C. §119(e) to U.S. Provisional Application Ser. No.61/783,441, entitled “Systems and Methods for Reducing Motion Blur inImages or Video in Ultra Low Light with Array Cameras” to Gabriel Molinafiled Mar. 13, 2013, the disclosures of which are hereby incorporated byreference in their entirety.

FIELD OF THE INVENTION

The present invention relates generally to capturing digital images andvideo and more specifically to the use of array cameras to reduce motionblur and/or noise when capturing images and video in low lightconditions.

BACKGROUND OF THE INVENTION

Low light image capture traditionally presents challenges in producingimages without excessive blurring or noise. Settings on a digital cameracan typically be adjusted to compensate for low light conditions. In adigital camera, individual image sensors corresponding to pixels in anoutput image receive light over a predetermined exposure time (alsocalled integration time). The exposure setting of an image sensor istypically the duration of time which light is sampled by individualpixel(s) in the image sensor. An analog gain is typically implementedthrough a circuit that amplifies the analog signal from a sensor beforeit is converted to a digital signal and processed. The exposure and gainsettings on image sensors in the camera are particularly relevant in lowlight conditions, as increases in exposure and gain generally increasethe voltage level of a pixel and thereby its apparent brightness. Underlow light conditions the use of a longer exposure time can provide abrighter image but may result in motion blur, where moving objects inthe scene are blurred because of movement over the time that lightassociated with those objects is being received by the camera.Increasing the gain can also provide a brighter image but can result inamplified noise artifacts.

SUMMARY OF THE INVENTION

Systems and methods for reducing motion blur in images or video in ultralow light with array cameras in accordance with embodiments of theinvention are disclosed. In one embodiment, a method for synthesizing animage from multiple images captured from different viewpoints using anarray camera includes capturing image data using active cameras withinan array camera, where the active cameras are configured to captureimage data and the image data captured by the active cameras includespixel brightness values that form alternate view images captured fromdifferent viewpoints, determining sets of corresponding pixels in thealternate view images where each pixel in a set of corresponding pixelsis chosen from a different alternate view image, summing the pixelbrightness values for corresponding pixels in the alternate view imagesto create pixel brightness sums for pixel locations in an output image,and synthesizing an output image from the viewpoint of the output imageusing the pixel brightness sums for the pixel locations in the outputimage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a conceptual illustration of an array camera architecture thatcan be used in a variety of array camera configurations in accordancewith embodiments of the invention.

FIG. 1A conceptually illustrates an optic array and an imager array inan array camera module in accordance with an embodiment of theinvention.

FIG. 2 is a conceptual illustration of a π filter arrangement that canbe used in a variety of array camera configurations in accordance withembodiments of the invention.

FIG. 3 is a flow chart showing a process for summing pixel brightnessvalues from multiple images obtained using an array camera in accordancewith an embodiment of the invention.

FIG. 4 is a flow chart showing a process for summed pixel brightnessvalues from image data captured from a reference viewpoint and one ormore alternate viewpoints in accordance with embodiments of theinvention.

DETAILED DISCLOSURE OF THE INVENTION

Turning now to the drawings, systems and methods for reducing motionblur in images or video in ultra low light with array cameras inaccordance with embodiments of the invention are illustrated. Arraycameras including camera modules that can be utilized to capture imagedata from different viewpoints are disclosed in U.S. patent applicationSer. No. 12/935,504, entitled “Capturing and Processing of Images usingMonolithic Camera Array with Heteregeneous Images”, filed May 20, 2009,the disclosure of which is incorporated by reference herein in itsentirety. Array cameras offer a number of advantages and features overlegacy cameras. An array camera typically contains two or more imagers(which can be referred to as cameras), each of which receives lightthrough a separate lens system. The imagers operate to capture imagedata of a scene from slightly different viewpoints. Array cameras have avariety of applications, including capturing image data from multipleviewpoints that can be used in super-resolution processing and depthcalculation. Imagers in the array may sense different wavelengths oflight (e.g., red, green, blue, Infrared) with the application ofselective filters, which can improve performance under differentlighting conditions and the performance of image processing processesperformed on image data captured using the array.

Array cameras in accordance with many embodiments of the inventionimprove the quality of images captured in low light conditions bysumming the brightness of corresponding pixels from different cameras.Low light image capture is particularly challenging because the exposuretime needed for a camera to receive a sufficient amount of light toproduce an image can result in motion blur. Alternatively, if gain isincreased to raise brightness levels, the noise level may be increasedcommensurately. In various embodiments of the invention, image data froma subset of cameras (imagers) in an array camera are chosen and pixelbrightness values of corresponding pixels in the image data are summed,producing higher brightness levels. By summing pixel brightness valuesfrom a pixel in a reference image with a corresponding pixel from one ormore alternate view image(s), the effective exposure time of the pixelin the reference image is increased by a factor equal to the number ofsummed pixels. Increasing exposure time can enable a reduction in analoggain and the associated noise. By exposing multiple pixels in parallel,the capture time can be significantly shorter than the effectiveexposure time of the pixel brightness values obtained in the referenceviewpoint by summing corresponding pixels. The opportunity for motionartifacts to be present in captured image data increases with increasedexposure time. Therefore, enabling an effective exposure time that issignificantly longer than the actual exposure time of the pixels in thereference camera will decrease the likelihood that motion artifacts willappear in the captured image data. In addition, providing an increasedeffective exposure time relative to the actual capture time can provideimproved low light video capture performance, where exposure time isconstrained by the frame rate of the video.

In a number of embodiments, image data captured by active cameras in anarray camera module is rectified (i.e. scene independent geometricshifts are applied to the image data captured by the cameras) and therectified image data is summed. In several embodiments, parallaxdetection and protection processes are performed to identify scenedependent geometric corrections to apply to the image data. Systems andmethods for performing parallax detection and correction are disclosedin U.S. Provisional Patent Application No. 61/691,666 entitled “Systemsand Methods for Parallax Detection and Correction in Imaged CapturedUsing Array Cameras” to Venkataraman et al. and U.S. Pat. No. 8,619,082entitled “Systems and Methods for Parallax Detection and Correction inImages Captured Using Array Cameras that Contain Occlusions usingSubsets of Images to Perform Depth Estimation” to Ciurea et al., thedisclosures of which are incorporated by reference herein in theirentirety. The pixel brightness values in the image data can be summedfollowing application of the scene dependent geometric corrections.Array cameras and methods for capturing images in low lightingconditions in accordance with embodiments of the invention are discussedfurther below.

Array Camera Architecture

An array camera architecture that can be used in a variety of arraycamera configurations in accordance with embodiments of the invention isillustrated in FIG. 1. The array camera 100 includes an imager array102, which is connected to a processor 104. Imagers 106 in the array 102are evenly spaced in a 5×5 square. In other embodiments, imagers mayhave different spacing or can be arranged in other orientations in thearray. The processor 104 is hardware, software, firmware, or acombination thereof that controls various operating parameters of theimager array 102. The processor 104 can also function to process theimages received from imager array 102 to produce a synthesized higherresolution image using super resolution processes, or transfer theimages to other hardware, software, firmware or a combination thereof toprocess the images. The system can also include memory 108 incommunication with the processor 104 for storing images. Architecturesfor imager arrays that can be utilized in accordance with embodiments ofthe invention include those disclosed in U.S. patent application Ser.No. 13/106,797, entitled “Architectures for System on Chip ArrayCameras” to Pain et al., the disclosure of which is incorporated hereinby reference in its entirety.

Although a specific architecture is illustrated in FIG. 1, any of avariety of architectures that enable the capture of low resolutionimages and application of super resolution processes to produce asynthesized high resolution image can be utilized in accordance withembodiments of the invention.

Array Camera Modules

Camera modules in accordance with many embodiments of the invention canbe constructed from an imager array and an optic array. A camera modulein accordance with an embodiment of the invention is illustrated in FIG.1A. The camera module 200 includes an imager array 230 including anarray of focal planes 240 along with a corresponding optic array 210including an array of lens stacks 220. Within the array of lens stacks,each lens stack 220 creates an optical channel that forms an image ofthe scene on an array of light sensitive pixels within a correspondingfocal plane 240. Each pairing of a lens stack 220 and focal plane 240forms a single camera 104 within the camera module. Each pixel within afocal plane 240 of a camera 104 generates image data that can be sentfrom the camera 104 to the processor 108. In many embodiments, the lensstack within each optical channel is configured so that pixels of eachfocal plane 240 sample the same object space or region within the scene.In several embodiments, the lens stacks are configured so that thepixels that sample the same object space do so with sub-pixel offsets toprovide sampling diversity that can be utilized to recover increasedresolution through the use of super-resolution processes.

In several embodiments, color filters in individual cameras can be usedto pattern the camera module with π filter groups as further discussedin U.S. Provisional Patent Application No. 61/641,165 entitled “CameraModules Patterned with pi Filter Groups”, to Nisenzon et al. filed May1, 2012, the disclosure of which is incorporated by reference herein inits entirety. These cameras can be used to capture data with respect todifferent colors, or a specific portion of the spectrum. In contrast toapplying color filters to the pixels of the camera, color filters inmany embodiments of the invention are included in the lens stack. Forexample, a green color camera can include a lens stack with a greenlight filter that allows green light to pass through the opticalchannel. In many embodiments, the pixels in each focal plane are thesame and the light information captured by the pixels is differentiatedby the color filters in the corresponding lens stack for each filterplane. Although a specific construction of a camera module with an opticarray including color filters in the lens stacks is described above,camera modules including π filter groups can be implemented in a varietyof ways including (but not limited to) by applying color filters to thepixels of the focal planes of the camera module similar to the manner inwhich color filters are applied to the pixels of a conventional colorcamera. In several embodiments, at least one of the cameras in thecamera module can include uniform color filters applied to the pixels inits focal plane. In many embodiments, a Bayer filter pattern is appliedto the pixels of one of the cameras in a camera module. In a number ofembodiments, camera modules are constructed in which color filters areutilized in both the lens stacks and on the pixels of the imager array.

In several embodiments, an array camera generates image data frommultiple focal planes and uses a processor to synthesize one or moreimages of a scene. In certain embodiments, the image data captured by asingle focal plane in the sensor array can constitute a low resolutionimage (the term low resolution here is used only to contrast with higherresolution images), which the processor can use in combination withother low resolution image data captured by the camera module toconstruct a higher resolution image through super-resolution processing.

A 4×4 array camera module including active cameras that capture imagedata used to synthesize an image from the viewpoint of a referencecamera in accordance with embodiments of the invention is illustrated inFIG. 2. The 4×4 camera module 250 includes an arrangement of cameraswith color filters such that 3×3 subsets of cameras are patterned usingπ filter groups. In the illustrated embodiment, a first π filter groupincludes a green camera at each corner, a green reference camera in thecenter indicated by a box 252, blue cameras above and below thereference camera, and red cameras to the left and right sides of thereference camera. In several embodiments, the locations of the red andblue cameras within the π filter group are swapped and/or an alternativecollection of cameras can be utilized to capture image data tosynthesize images. In various embodiments, a second subset of activecameras includes a row of blue, green, and red cameras placed below theπ filter group and a column of blue, green, and red cameras placed tothe right side of the π filter group with a green camera connecting therow and the column. Although all of the cameras in the array cameramodule illustrated in FIG. 2 are shown as capturing image data, in manyembodiments one or more of the cameras within the array camera modulecan be idle during image capture to conserve power as appropriate to therequirements of a specific application.

In many embodiments of the invention, one camera is designated as areference camera capturing image data from a reference viewpoint and anumber of cameras with the same color filter as the reference camera aredesignated alternate view cameras (that capture image data from slightlydifferent viewpoints). In the embodiment illustrated in FIG. 2, a greencamera 252 is chosen as reference camera and a plurality of other greencameras in the array including second and third green cameras 254 and256 are chosen as alternate view cameras. The alternate view cameras caninclude all other green cameras or a subset of the other green camerasin the array. As will be discussed in greater detail further below, thebrightness values of pixels in the alternate view cameras can be summedwith the brightness values of pixels in the reference camera to increasethe effective exposure time of the pixels relative to the image datacapture time.

In various embodiments of the invention, an array can include multiplereference cameras with multiple subsets of alternate view cameras thatcan be used to synthesize images with summed pixel brightness valuesfrom different viewpoints. The diversity of image data from differentviewpoints can be used in various applications such as synthesizinghigher resolution images with super-resolution processes. In severalembodiments of the invention, a green camera indicated by a box 258 isalso a reference camera. Image data from alternate view green cameras(that can be different from the alternate view cameras associated withcamera 252) are combined with image data from green camera 258 tosynthesize image data representative of an image from the viewpoint ofthe green camera 258. The image data generated from the viewpoint of thegreen camera 258 can be used in combination with the image datagenerated from the viewpoint of the green camera 252 in applicationsthat utilize multiple images from different viewpoints such assuper-resolution processes.

Although specific array camera module configurations and partitions ofcameras into subsets for synthesizing images are discussed above withrespect to FIG. 2, partitions of active cameras into subsets for thepurpose of capturing image data for synthesizing images can be utilizedwith any of a variety of camera module configurations such as, but notlimited to, array camera modules including any number of cameras, arraycamera modules in which one or more of the cameras capture image datawithin multiple color channels (i.e. individual cameras in the array arepossibly non-monochromatic), and/or array camera modules in which notall cameras of the camera array are active (i.e. not capturing imagedata or performing measurements) as appropriate to the requirements of aspecific application in accordance with embodiments of the invention.Processes for utilizing array camera architectures with color filtersfor reducing motion blur in low light conditions is discussed below.

Reducing Motion Blur by Summing Pixel Brightness Values

As explained further above, traditional techniques to increase signallevels when capturing a digital image using image sensors includechanging the exposure or gain, with some negative effects in turn. Whenusing an array camera to capture multiple images of the same scene inthe same moment an additional factor is available. By adding pixelvalues/signal levels together between corresponding pixels in thecaptured image data, a higher “boosted” signal level can be achievedwith less of the negative effects of modifying the exposure and/or gainof the cameras. Furthermore, this high signal level can be achievedwhile lowering exposure and/or gain to decrease their negative effects.

In many embodiments of the invention, a subset of cameras in the arrayis chosen. Pixel values in a number of images captured by the selectedcameras are combined, resulting in a pixel brightness sum correspondingto an effective exposure time equal to the number of summed pixelsmultiplied by the actual image data capture time. In further embodimentsof the invention, because the pixel values are increased by the numberof cameras from which pixel values are summed, the gain and/or exposuretime of each of the cameras can be reduced accordingly.

A process for generating an image from summed pixel values from multipleimages captured by an array camera in accordance with embodiments of theinvention is illustrated in FIG. 3. The process 300 includes determining(310) image capture settings for active cameras in an array. Imagecapture settings can include (but are not limited to) exposure time,analog gain, and frame rate (when capturing video). As will be discussedfurther below, these settings can be adjusted based on the number ofactive cameras used.

Image data is captured (320) using a set of active cameras in the array.Typically, each camera produces an image from its point of view and theimage data forms images made up of pixel brightness values. In arraycameras, often one camera is designated a reference camera and the imagedata captured by that camera is referred to as being captured from areference viewpoint. In several embodiments of the invention, the set ofactive cameras is chosen where the cameras have color filters such thatthey capture information within the same spectral band (also referred toas color channel). A spectral band can correspond to a human perceptiblecolor (e.g., red, green, blue) or can be non-perceptible (e.g.,infrared). As discussed further above, an array camera may utilize afilter pattern such that it contains cameras that separately capturegreen, red, and blue light (designated green, red, and blue camerasrespectively). Therefore, a set of active cameras can include allcameras of one color, such as green cameras.

The pixel brightness values of corresponding pixels in the image datacaptured from alternate viewpoints are summed (340). Correspondingpixels can refer to pixels that represent the same scene content in eachimage. Parallax, due to the different fields of view of the activecameras, can affect pixel correspondence. Some pixels that capture thesame portion of a scene (i.e. pixel brightness values corresponding tosamples of corresponding portions of the object space) may be indifferent locations in different images due to parallax. In manyembodiments of the invention, images are compensated (330) for parallax.Parallax compensation such as applying scene-dependent geometric shiftsto the affected pixels is discussed further below. Images can also becompensated for distortions due to the physical characteristics of theparticular imager (also referred to as geometric compensation orcorrection) using scene-independent geometric shifts to aligncorresponding pixels.

In many embodiments of the invention, one image is designated areference image and the other images in the set of images are referredto as alternate view images. The designation of a reference image hasparticular relevance in determining a reference viewpoint andcompensating for parallax. Where one image is designated a referenceimage, the pixels of the reference image are summed with correspondingpixels in alternate view images. In other embodiments, the referenceimage may be a ‘virtual’ image synthesized in a location in the arraywhere no physical camera exists. When a ‘virtual’ image is used, thecorresponding pixels from alternate view images can be summed into thegrid for a reference viewpoint where image data may not physicallyexist. Systems and methods for generating a ‘virtual’ image from theperspective of a given viewpoint include, but are not limited to, thosedisclosed in U.S. Provisional Application No. 61/707,691 entitled“Synthesizing Images From Light Fields Utilizing Virtual Viewpoints” toJain, filed Sep. 28, 2012, U.S. application Ser. No. 14/042,275 entitled“Generating Images from Light Fields Utilizing Virtual Viewpoints” toNisenzon et al., filed Sep. 30, 2013, U.S. Provisional Application No.61/776,751 entitled “Systems and Methods for Image Data Compression” toMcMahon et al., filed Mar. 11, 2013, and U.S. application Ser. No.14/204,990 entitled “Systems and Methods for Image Data Compression” toMcMahon et al., filed Mar. 11, 2014, the disclosures of which are herebyincorporated by reference in their entirety.

An output image is synthesized (350) from the pixel brightness sums.Where a reference image has been designated, values equal to the pixelbrightness sums corresponding to pixels in the reference image can beplaced in the same locations in the output image. Where multiplereferences images have been designated, super- resolution processes canbe utilized to synthesize a higher resolution image using the pixelbrightness sums determined for each of the reference images. In otherwords, multiple independently summed images can be used as inputs to asuper-resolution process to generate a higher resolution image.

In further embodiments of the invention, image capture settings such asexposure time and gain can be adjusted in view of the higher signallevels resulting from the summation. For example, presume image data iscaptured using a 4×4 array where eight cameras have green filters. Imagedata can be combined by summing the corresponding pixels from the eightgreen cameras. Given that that there are eight active green cameras, thepixel brightness values are approximately eight times higher than thepixel brightness values of an image from a single green camera. Underthese conditions, the exposure time can be reduced by eight times,having the effect of maintaining the same apparent brightness and noisewhile reducing the apparent motion blur. Alternatively, the analog gainof cameras can be reduced by eight times, having the effect of reducingthe apparent noise in the image while maintaining the same brightness,or the exposure time and the analog gain can each be reduced by anamount (such as half exposure and one quarter analog gain) so that thereis a total reduction by a factor of eight.

If multiple images are captured over time to generate a video sequence,the frame rate (i.e., rate at which images are captured) can beadjusted. The theoretical maximum exposure time is the inverse of framerate. As exposure time is decreased, the frame rate can be increasedaccordingly. Image capture settings such as exposure time, gain, andframe rate can be determined (310) before the images are captured.Although specific processes for increasing the effective exposure timesof pixels in a reference image relative to the actual image data capturetime by a factor equal to the number of summed pixels are discussedabove with reference to FIG. 3, any of a variety of processes can beutilized to increase effective exposure times of pixels in a referenceimage by summing the pixel brightness values in the reference image withthe pixel brightness values of corresponding pixels in alternate viewimages can be utilized as appropriate to the requirements of specificapplications in accordance with embodiments of the invention. Theeffects of parallax and techniques to compensate for parallax whensumming corresponding pixels to increase effective pixel exposure timesin accordance with embodiments of the invention are discussed below.

Disparity and Compensating for Parallax

Images of a scene captured by different cameras in an array camera canhave slight differences due to the different fields of view resultingfrom the different locations of the cameras, an effect known asparallax. These differences, referred to as disparity, provideinformation that can be used to measure depth of objects within a scene.Once depth information has been determined, scene-dependent geometricshifts can be applied to the pixels of the captured images to remove thedifferences in the images that resulted from parallax. The modifiedimages then have similar pixels, corresponding to the same observedpoints in the scene, in the same locations. Systems and methods fordetecting and correcting parallax are discussed in U.S. patentapplication Ser. No. 61/691,666 entitled “Systems and Methods forParallax Detection and Correction in Images Captured Using ArrayCameras” to Venkataraman et al. and U.S. Pat. No. 8,619,082, thedisclosures of which are incorporated by reference above.

Techniques such as those disclosed in the patent applicationincorporated above are typically used to generate a depth map from areference viewpoint. The reference viewpoint can be from the viewpointof one of the cameras in a camera array. Alternatively, the referenceviewpoint can be an arbitrary virtual viewpoint. A depth map indicatesthe distance of the surfaces of scene objects from a reference viewpointand can be utilized to determine scene dependent geometric correctionsto apply to the pixels from each of the images within captured imagedata to eliminate disparity when fusing images together as insuper-resolution processing (generating a higher-resolution image frommultiple lower-resolution images) and/or when summing pixel brightnessvalues for corresponding pixels.

Corrections for parallax may be desired when parallax results in scenedifferences in the images used for corresponding pixel summation. Inseveral embodiments of the invention, the processes discussed above canbe utilized without parallax correction where no objects are within acertain distance or where the minimum depth at which objects appear inthe image (or equivalently, disparity) is determined to be below acertain threshold. For example, sparse depth information can begenerated for a reference image and the pixel brightness values in thereference view image data are summed with pixel brightness values inalternate view image data when no objects are within a thresholddistance according to the generated depth information. In furtherembodiments of the invention, images can be compensated for parallax inall situations or where the depth of objects in the image is determinedto be above a predetermined threshold.

A process for generating summed pixel values from image data capturedfrom a reference viewpoint and one or more alternate viewpoints, whereparallax detection and correction is utilized to identify correspondingpixels within the image data in the in accordance with embodiments ofthe invention is illustrated in FIG. 4. Similar to the process describedabove with respect to FIG. 3, image capture settings can be determined(410) and image data captured (420). Depth measurements are calculatedusing at least a portion of the image data (430) and a determination ismade whether any objects in the scene are within a predeterminedthreshold depth/distance from the camera (440). If no objects are withinthe predetermined threshold, pixel brightness values are summed (450)and an output image generated (460). If there is at least one objectwithin the threshold distance, parallax correction is performed.Techniques for correcting for parallax can be utilized in a variety ofways. Processes such as those disclosed in U.S. Provisional PatentApplication No. 61/780,974 entitled “Systems and Methods forSynthesizing Images from Image Data Captured by an Array Camera UsingDepth Maps in which Depth Estimation Precision and Spatial ResolutionVary” to Venkataraman et al. and U.S. patent application Ser. No.14/207,254 entitled “Systems and Methods for Synthesizing Images fromImage Data Captured by an Array Camera Using Restricted Depth of FieldDepth Maps in which Depth Estimation Precision Varies” to Venkataramanet al. can be utilized to correct parallax in images before pixelsumming as discussed below. The disclosures of U.S. patent applicationSer. Nos. 61/780,974 and 14/207,254 are hereby incorporated by referencein their entirety. In many embodiments of the invention, parallaxdetection and compensation includes one or more modes as will bediscussed below.

In a first mode of operation, referred to here as user plane focus mode,parallax detection and correction is only performed with respect topixel brightness values that sample objects in the scene that are at aspecific focus depth and/or within a specific depth of field relative tothe focus depth. The focus depth can be determined from at least onedesignated region of interest within the captured image and/or a previewimage. The region of interest can be a predetermined location (e.g., anauto-focus rectangle in the center of the image) or can be selected by auser in real time. A sparse depth map can be created with higherresolution with respect to regions of the image containing objects thatare located within the specified depth of field relative to thespecified focus depth. Systems and methods for generating sparse depthmaps include, but are not limited to, those disclosed in U.S. patentapplication Ser. Nos. 61/780,974 and 14/207,254 incorporated byreference above. In some embodiments, the depth detection might occurwith denser depth sampling in the desired depth-of-field and less densedepth sampling outside the desired depth-of-field. In other embodiments,the depth of field may be set to be extremely small such that the depthdetection might be entirely skipped and the depth map may be assumed toconsist only of pixels at the target depth. Using the depth map, thedisparity between a reference viewpoint and the alternate viewpoints canbe determined (470) using the baseline between each of the alternateview cameras and the reference camera. The disparity can then be used toidentify (480) corresponding pixels within the image data based upon theanticipated scene-dependent geometric shifts present in the image data.In this way, the pixels in the reference plane that are in focus aresummed using corresponding pixels identified in a manner that accountsfor disparity between the reference viewpoint and an alternateviewpoint. In this way, the parallax detection and correction processcan reduce any blur that may be introduced by summing pixels that areincorrectly identified as corresponding. After parallax compensation,objects in the target focus depth of field will be aligned across thealternate images and summing the pixels containing those objectsprovides a higher effective exposure in the final image for thoseobjects. Pixels containing objects not at the target focus depth may notbe aligned across the images and summing those pixels may result inblurring. However, there may already be blurring due to the target focusnot being set for those objects (they are “out of focus”) so theadditional blurring may not be visually significant.

In a second mode, referred to here as dense parallax compensated mode,parallax correction is performed for all pixels in an image before beingsummed. In this mode, a depth map is calculated for all pixels in areference image and corresponding pixels in the alternate images areidentified based upon the scene-dependent geometric shifts (470) (480)with respect to the reference image predicted by the depth map.

In a third mode, all pixels in an image are compensated before beingsummed. However, the same compensation is applied to all pixelsirrespective of their depth indicated on a depth map. The uniformcompensation can be based on a chosen depth. In many embodiments, thedepth can be determined by identifying a region of interest (usingtechniques such as those discussed above) and calculating a depth map ofthe region. In several embodiments, a histogram of the depths in theregion is formed. In some embodiments, the histogram can be filtered toeliminate low confidence regions of the depth map such as texturelessregions or to admit only high confidence regions such as edges. In manyembodiments, the median depth of the histogram is taken to be thedesired depth of focus. Systems and methods for determining a depth offocus include, but are not limited to, those disclosed in U.S. patentapplication Ser. Nos. 61/780,974 and 14/207,254 incorporated byreference above. The depth is then used to compensate all pixels beforesumming (can be seen as equivalent to setting a depth map to a fixeddepth everywhere). This will tend to attenuate regions of the image thatare off the desired focal depth (not aligned) and amplify regions whichare on or close to the focal depth (aligned). For objects in thereference viewpoint which are actually at or near the target depth,corresponding pixels will naturally align and the summed image will havean appearance of sharpness. For objects in the reference viewpoint whichare not actually at or near the target depth, the resulting summed pixelwill be an average of many non-corresponding pixels. The resulting imagewill typically be a synthetic aperture image where objects at the targetdepth will appear sharper and brighter than objects far from the targetdepth, which will appear blurred. This mode can allow reducedcomputation in situations where parallax processing is toocomputationally demanding, or in applications where the scene content istypically at a fixed depth (such as usually beyond a certain distance).In many embodiments, once the final image is summed it is divided ormultiplied by a scale factor as needed to set a desired targetbrightness for final output.

When image data is captured from different perspectives and the sceneincludes foreground objects, the disparity in the location of theforeground object in each of the images results in portions of the scenebehind the foreground object being visible in some but not all of theimages. A pixel that captures image data concerning a portion of ascene, which is not visible in images captured of the scene from otherviewpoints, can be referred to as an occluded pixel. These occlusionscan be detected by determining whether there is a great differencebetween pixels that should correspond according to depth map anddisparity calculations. In many embodiments, at least some portions ofimages that have these occlusions are not included in the summation toavoid creating artifacts. Systems and methods for detecting occlusionsand correcting for parallax include, but are not limited to, thosedescribed in U.S. patent application Ser. No. 61/691,666 and U.S. Pat.No. 8,619,082 incorporated by reference above.

In several embodiments, where occluded pixels are detected, only thosepixels corresponding to pixels visible in the reference image can beused in the summation. In many embodiments, the occluded pixels can beleft out of the summation. If pixels are left out of the summation, thetotal of the pixels that are summed should be scaled by a factor tomatch the brightness of the other summed pixels. The factor can bedetermined by dividing the total number of images that could potentiallyinclude the pixel by the number of cameras that actually observe thatpixel. For example assume N_(g) cameras are summed for areas of an imagewhere all pixels are visible and N_(gv) cameras have visibility ofcertain pixels in an occluded area. Those pixels with visibility aresummed and the sum is multiplied by a factor of N_(g)/N_(gv) tocompensate for the pixels left out.

Although specific techniques for parallax compensation are discussedabove with respect to processes for summing pixel brightness values, anyof a variety of processes can be utilized to correct parallax inaccordance with embodiments of the invention. Temporal framecompensation in accordance with embodiments of the invention isdiscussed below.

Motion-Compensated Temporal Filtering

Noise and signal performance of array cameras in low light conditionscan be further improved by temporal frame compensation. Multiple framesare captured of a scene over time (e.g., frames captured at times N−1,N, and N+1). For each pixel a motion compensation vector is determinedbetween each frame. The motion compensation vector can be calculatedbetween individual camera images captured for each frame or betweensummed images formed from the images corresponding to each frame (thatis, summed image at N−1, summed image at N, and image at N+1). Using themotion compensation vectors, the brightness values of correspondingpixels between frames are added to generate an image representing thesum of images from multiple cameras and multiple frames. This summationcan be divided to produce an average (which may tend to reduce noise) orcan be compensated for by reducing exposure time (which may tend toreduce motion blur). Producing a color image from image data capturedwithin discrete narrow spectral bands in accordance with embodiments ofthe invention is discussed below.

Combining Image Data Captured in Narrow Spectral Bands into a ColorImage

When utilized with sets of cameras that capture a single color, thetechniques discussed above generally produce a monochrome image in thatcolor. In many embodiments of the invention, a composite color RGB imagecan be formed from monochrome images of different colors (i.e., separatecolor channels). As discussed above, a monochrome image can be formed bysumming pixel brightness values of images from a set of cameras with thesame color filter in an array. The array can contain sets of greencameras, red cameras, and blue cameras. Recognizing that the number ofcameras in each set may not be equal, the summed pixel brightness valuescan be normalized across the sets of cameras and the summed images canbe combined into a color image.

Assuming the set of green cameras has the largest number of cameras, theimages from the sets of red and blue cameras can be normalized asfollows. A green image is generated using the processes described aboveto sum pixel brightness values from green cameras. Similarly, a redimage is generated from the red cameras and a blue image is generatedfrom the blue cameras. The pixel brightness values of the red image areincreased by a factor N_(g)/N_(r) where N_(g) is the number of greencameras and N_(r) is the number of red cameras. Similarly, the pixelbrightness values of the blue image are increased by a factorN_(g)/N_(b) where N_(b) is the number of blue cameras. The images canthen be combined into a color image where the relative color levels arecorrect with respect to each other. Increasing the signal levels fornormalization can be accomplished in a variety of ways includingincreasing the analog gain of cameras or multiplying the signal in thedigital domain after analog-to-digital conversion.

Although the present invention has been described in certain specificaspects, many additional modifications and variations would be apparentto those skilled in the art. It is therefore to be understood that thepresent invention may be practiced otherwise than specificallydescribed, including various changes in the implementation such asutilizing encoders and decoders that support features beyond thosespecified within a particular standard with which they comply, withoutdeparting from the scope and spirit of the present invention. Thus,embodiments of the present invention should be considered in allrespects as illustrative and not restrictive.

What is claimed is:
 1. A method for synthesizing an image from multipleimages captured from different viewpoints using an array camera in lowlight conditions, the method comprising: capturing image data using aplurality of active cameras within an array camera, where the pluralityof active cameras are configured to capture image data within the samespectral band and the image data captured by the active camerascomprises pixel brightness values that form a reference image and aplurality of alternate view images captured from different viewpoints;applying geometric shifts to shift the plurality of alternate viewimages to the viewpoint of the reference image using the processorconfigured by software; summing the pixel brightness values for pixelsin the reference image with pixel brightness values for correspondingpixels in the alternate view images to create pixel brightness sums forthe pixel locations in the reference image using the processorconfigured by software; and synthesizing an output image from theviewpoint of the reference image using image data comprising the pixelbrightness sums for the pixel locations in the reference image using theprocessor configured by software.